Investments (Analysis of Past Due Commercial Mortgage, Agricultural and Other Loans) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 1,379,635	$ 1,389,662
Total Loans	1,379,635	1,389,662
Non-Accrual Status	0	0
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,324,462	1,330,465
Total Loans	1,324,462	1,330,465
Non-Accrual Status	0	0
Commercial Mortgage Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial Mortgage Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Commercial Mortgage Loans | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	55,173	59,197
Total Loans	55,173	59,197
Non-Accrual Status	0	0
Agricultural Property Loans | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Agricultural Property Loans | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	0	0
Loans | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past Due	$ 0	$ 0